The CASSIOPES project has two main goals. On the one hand, the first purpose of this project is to fabricate a functional 3D Li-ion all solid state micro-battery by using thin film technologies allowing to reach high energy density (> 5 mW.cm-2). 3D Lithium ion technology is proposed in order to find a suitable alternative to commercially available planar metallic lithium micro-battery. To fabricate the 3D prototype, Atomic Layer Deposition facility has been selected as the thin film deposition tool. On the other hand, all the thin films within the micro-battery (electrodes, electrolyte, current collectors, seed layer…) will be studied in order to investigate and to understand the charge storage mechanism in a dynamic way during cycling by using advanced in situ/operando characterization technique available within the consortium. The energy density of the 3D Li-ion micro-battery technology proposed in the frame of the CASSIOPES project will be 5 up to 10 times higher than the planar technology. Moreover, CASSIOPES aims at breaking technological barriers as proposed as strategic direction by the SNR (instrumentation, in situ/operando characterization, study of interface at the nanoscale – 15th orientation). The in situ/operando characterization tool developed within CASSIOPES project has already used in bulk electrodes or batteries but should be revised in order to match with the problematic of thin films technology for electrochemical energy storage. Those techniques has been listed as priority activity of the “Defi 3 – axe4”.